Control system for mobile NOx SCR applications

ABSTRACT

A diesel powered vehicle is provided with an SCR system which uses an external reducing reagent to convert NOx emissions in a manner which accounts for the effects of NOx transient emissions on the reducing catalyst. Actual NOx emissions produced by the engine are filtered using a variable NOx time constant in turn correlated to the reductant/NOx storage capacity of the reducing catalyst at its current temperature to account for changes in the SCR system attributed to NOx transient emissions. Catalyst temperature is filtered using a variable catalyst time constant corresponding to current space velocity of the exhaust gas to account for changes in the catalyst temperature attributed to NOx transient emissions. The reductant is metered on the basis of the filtered, corrected NOx concentration applied at a NSR ratio based, in turn, on the filtered, corrected reducing catalyst temperature.

This invention relates generally to nitrogen oxide (NOx) emissionsproduced by internal combustion engines in a vehicle and moreparticularly to a system for controlling reduction of the NOx emissionsby means of a selective catalytic reduction (SCR) method.

The invention is particularly applicable to and will be described withspecific reference to a control system for regulating the supply of anexternal reductant, ammonia, to a reducing catalyst in an SCR systemtaking into account the effect of NOx transient emissions produced invehicles powered by diesel engines. However, those skilled in the artwill recognize that the control system has broader applications and maybe applied to SCR systems using other reductants such as fuel oil orhydrocarbons as well as SCR systems used in other mobile internalcombustion engine applications, such as gasoline engines employing “leanburn” techniques.

INCORPORATION BY REFERENCE

The following patents and publications are incorporrated by referenceherein and made a part hereof:

-   -   1) U.S. Pat. No. 4,403,473, to John R. Gladden; dated Sep. 13,        1983, entitled: “AMMONIA/FUEL RATIO CONTROL SYSTEM FOR REDUCING        NITROGEN OXIDE EMISSIONS”;    -   2) SAE Paper No. 952493, by H. Luders, R. Backes, and G.        Huthwohl, FEV Motoremtechnik and D. A. Ketcher, R. W.        Horrocks, R. G. Hurley and R. H. Hammerle, Ford Motor Co., dated        Oct. 16–19, 1995, entitled: “AN UREA LEAN NOx CATALYST SYSTEM        FOR LIGHT DUTY DIESEL VEHICLES” (See page 7);    -   3) SAE Paper No. 921673, by J. Walker, Ortech and B. K.        Speronello, Engelhard Corp., dated Sep. 14–17, 1992, entitled:        “DEVELOPMENT OF AN AMMONIA/SCR NOx REDUCTION SYSTEM FOR A HEAVY        DUTY NATURAL GAS ENGINE”;    -   4) U.S. Pat. No. 5,606,855, to Naoki Tomisawa, dated Mar. 4,        1997, entitled: “APPARATUS AND METHOD FOR ESTIMATING THE        TEMPERATURE OF AN AUTOMOTIVE CATALYTIC CONVERTER”; and,    -   5) U.S. Pat. No. 5,490,064, to Minowa et al., dated Feb. 6,        1996, entitled: “CONTROL UNIT FOR VEHICLE AND TOTAL CONTROL        SYSTEM THEREFOR”.

None of the material cited above form any part of the present invention.The material is incorporated by reference herein so that detailsrelating to SCR systems such as the operation of the SCR systems withammonia or hydrocarbon reductants, the metering, of the reductants,principles and control of engine operation etc., need not be set forthor described in detail herein.

BACKGROUND

This invention is directed to the removal of nitrogen oxides (NOx) fromthe exhaust gases of internal combustiobn engine, particularly dieselengines, which operate at combustion conditions with air in large excessof that required for stoichiometric combustion, i.e., lean.Unfortunately, the presence of excess air makes the catalytic reductionof nitrogen oxides difficult. Emission regulations impose a limit on thequantity of specific emissions, including NOx, that a vehicle can emitduring a specified drive cycle, such as i) for light duty trucks, an FTP(“federal test procedure”) in the United States or an MVEG (“mobievehicle emissions group”) in Europe or ii) for heavy duty trucks, aHeavy Duty Cycle in the United States or an ESC (European Steady StateCycle) or ETC (European Transient Cycle) in Europe. The regulations areincreasingly limiting the amount of nitrogen oxides that can be emittedduring the regulated drive cycle.

There are numerous ways known in the art to remove NOx from a waste gas.This invention is directed to a catalytic reduction method for removingNOx. A catalytic reduction method essentially comprises passing theexhaust gas over a catalyst bed in the presence of a reducing gas toconvert the NOx into nitrogen. Conventionally, there are three ways totreat vehicular exhaust to reduce NOx. The first method is non-selectivecatalyst reduction (NSCR). The second way is selective non-catalyticreduction (SNCR) and the last method is selective catalyst reduction(SCR). This invention relates to SCR systems.

In diesel engines, sufficient NOx reduction to meet current regulationshas been achieved by combustion modifications in the diesel engine by,for example, incorporating EGR. Projected emission levels are such thatcombustion and engine modifications will not be sufficient to meet themore stringent levels. Because of excess oxygen present in dieselexhaust gases, the opportunity for NOx reduction under rich orstoichiometric air/fuel is not possible. SCR is a technology that hasbeen shown effective in removing NOx from oxygen rich exhaust. A numberof SCR systems have been developed which, because of infrastructureconcerns, have used diesel fuel or diesel oil as the reductant source.Unfortunately, as of this date, an HC reducing catalyst has not yet beendeveloped which has sufficient activity and is effective over the entireoperating range of the diesel engine.

A common nitrogen oxide reducing agent, long used in industrialprocesses, is ammonia. NOx reducing catalysts have been developed whichare effective over the operating range of the engine. Despite theinfrastructure concerns relating to the use of urea in a mobileapplication as well as the potentially dangerous risks of ammoniabreak-through or slip, ammonia SCR systems are becoming the favoredchoice for mobile applications to meet the more stringent NOx emissions.This is, among other reasons, because of the high NOx conversionpercentages possible with ammonia coupled with the ability to optimizethe combustion process for maximum power output with minimum fuelconsumption.

Notwithstanding what may be said to be inherent advantages of an ammoniabased SCR system, the control systems to date have been excessivelycomplicated and/or ineffective to control the SCR system when the impactof NOx transient emissions on the SCR system is considered. As will beshown below, if the transient NOx emissions can not be adequatelyreduced by the SCR control system, then stringent NOx emissionregulations will not be met.

Early patents controlled ammonia metering by considering the emissionsto be controlled at steady state conditions. For example, U.S. Pat. No.4,403,473 to Gladden (Sep. 13, 1983) considered NOx emissions at variousspeed ranges and concluded that a linear relationship exists betweenfuel flow and NOx. (Earlier Gladden U.S. Pat. No. 4,188,364, Feb. 12,1980 concluded that ammonia catalyst adsorbed ammonia at temperatureslower than 200° C. and desorbed at temperatures between 200–800° C., theSCR system should operate at higher, temperatures to achieve completereaction between ammonia and NOx.) Thus, in Gladden '473, the fuel massflow is sensed and NH₃ throttled at a percentage of fuel flow providedthe temperature of the gases in the catalytic converter are within a setrange. This basic control concept is used today in most mobile, ammonia,SCR systems. For example, U.S. Pat. No. 5,116,579 to Kobayashi et al.(May 26, 1992) additionally measures the humidity of intake air and oneor more operating parameters of engine power, intake air temperature,fuel consumption and exhaust gas temperature to set an ammonia ratiocontrol valve. The molar ratio of ammonia to NOx is set at less than one(sub-stoichiometric) to minimize ammonia slip.

Typically the reductant is pulse metered into the exhaust gas stream ina manner similar to that used for operating conventional fuel injectors.In U.S. Pat. No. 4,963,332 to Brand et al. (Oct. 16, 1990), NOx upstreamand downstream of the catalytic converter is sensed and a pulsed dosingvalve controlled by the upstream and downstream signals. In U.S. Pat.No. 5,522,218 to Lane et al. (Jun. 4, 1996), the pulse width of thereductant injector is controlled from maps of exhaust gas temperatureand engine operating conditions such as engine rpm, transmission gearand engine speed.

As noted, the industrial art has long used ammonia in SCR systems tocontrol NOx emissions typically by set point control loops such as shownin U.S. Pat. No. 5,047,220 to Polcer (Sep. 10, 1991) in which adownstream NOx sensor is used to generate a trim signal in the controlloop. The industrial art has also recognized that changes in load fromthe turbine, furnace etc. affects the ammonia SCR systems. Thus in U.S.Pat. No. 4,314,345 to Shiraishi et al. (Feb. 2, 1982), variations inload are determined by sensing the temperature of the exhaust gas. Whenthe exhaust gases are at certain temperature ranges a variation in theload is assumed to occur and different or predicted NH3/NOx molar ratiosare used to account for the adsorption/desorption characteristics of thecatalyst. A more sophisticated molar ratio control system is disclosedin U.S. Pat. No. 4,751,054 to Watanabe. Watanabe uses not only upstreamand downstream NOx sensors but also temperature, flow rate and NH3detectors to set a mole ratio correcting signal. In U.S. Pat. No.4,473,536 to Carberg et al. (Sep. 25, 1984) a turbine's inlet airflow,discharge pressure, discharge temperature and mass fuel flow are sensedto predict NOx generated by the turbine which signal is corrected forNOx error by time delayed NOx sensor measurements. Carberg recognizesthat turbine load changes may change NOx emissions in a time framequicker than the 1 plus minute needed to determine the NOx emissions ina gas sample with conventional NOx sensors and thus makes a prediction,which can not be corrected in real time. The industrial systems, for themost part, do not operate under the highly transient conditions whichcharacterize vehicle engines producing sudden NOx transients. Industrialsystems also operate in an environment in which samples of the gas beingproduced can be taken to accurately determine the NOx content to trimthe ammonia metering valve in closed loop control.

In addition to systems which sense engine operating parameters tocontrol metering of ammonia or a reductant, there are other approachesused to control NOx emissions in mobile applications. In U.S. Pat. No.5,845,487 to Fraenkle et al. (Dec. 8, 1998), the exhaust gas temperatureis sensed. If the exhaust gas is outside the temperature limits at whichthe SCR system is effective i.e., below the operating temperature, thefuel injection timing to the engine is retarded, reducing the NOx viacombustion modifications. In U.S. Pat. No. 5,842,341 to Kibe (Dec. 1,1998) space velocity and exhaust gas temperature is measured todetermine the reductant quantity. In addition inlet and outlet catalyticconverter temperature is measured and reductant flow is decreased fromthe steady state conditions when the temperature differential betweeninlet and outlet begins to increase. The reductant, disclosed as HC inKibe's preferred embodiment, does not according to Kibe otherwisecontribute, by exothermic HC oxidation reactions, to heating of thecatalyst mass or bed. The reductant is decreased to keep the catalystwithin the operating temperature window.

Perhaps one of the more sophisticated approaches to using urea/ammoniasystem in a mobile application is disclosed in a series of patents whichinclude U.S. Pat. No. 5,833,932 to Schmelz (Nov. 10, 1998); U.S. Pat.No. 5,785,937 to Neufert et al. (Jul. 28, 1998); U.S. Pat. No. 5,643,536to Schmelz (Jul. 1, 1997); and U.S. Pat. No. 5,628,186 to Schmelz (May13, 1997). While these patents discuss reducing reagents in a generalsense, they are clearly limited to urea/ammonia reductants. According tothis system, a catalytic converter having composition defined in the'932 patent, has a reducing agent storage capacity per unit length thatincreases in the direction of gas flow. This allows for positioning ofinstrumentation along the length of the catalyst as disclosed in the'536 patent to determine the quantity of ammonia stored in the catalyst.The catalyst is charged with the reducing agent such that transientemissions can be converted by the reducing agent stored in the catalyticconverter. The '186 patent, however, is directed as is the presentinvention, to a control system not limited to any specific catalyst. The'186 patent recognizes, as does several prior art references discussedin this section, that i) sudden increases in load require a decrease inthe reducing agent (and similarly sudden decreases in load require anincrease in the reducing agent) and ii) the temperature (the '186 patentalso requires exhaust gas pressure) of the reducing catalyst affects itsability to store and release the reducing agent. The '186 patentmeasures, from changes in gas pressure and catalyst temperature, therate at which the reducing agent is being adsorbed or desorbed from thecatalyst. It then calculates NOx emissions produced from the engine andsets a sub-stoichiometric ratio of reducing agent/NOx emissions at whichthe reducing agent is metered to the catalyst. The metering reducingagent rate is then adjusted upward or downward to equal the measuredrate of reducing agent adsorption/desorption. A burner is provided to“empty” the catalyst apparently to assure a sound reference value uponengine start for measurements and to guard against slip. Assuming theadsorption/desorption theory and measurement capability is “sound”, thesystem is sound although a large number of sensors and intensivecalculations appear to be required.

Within the literature, a significant number of articles have beenpublished investigating ammonia SCR NOx reducing systems and severalarticles have discussed control strategies to optimize the SCR NOxsystems investigated. In SAE paper 921673, entitled “Development of anAmmonia/SCR NOx Reduction System for a Heavy Duty Natural Gas Engine” byJ. Walker and B. K. Speronello, (September 1992), various quantities ofammonia were injected at various engine speeds and loads to obtainoptimum NOx, conversions at steady state engine speeds and loads. Thespeeds and loads were mapped and stored in a look-up table (specific foreach engine and each SCR catalyst) which was then accessed periodicallyto set an ammonia metering rate. This open loop, feed forward techniqueis conventionally used and produces good conversion ratios for steadystate conditions.

SAE paper 970185, “Transient Performance of a Urea deNOx Catalyst forLow Emissions Heavy-Duty Diesel Engines” by Dr. Cornelis Havenith andRuud P. Verbeek (a co-inventor of the subject application) datedFebruary, 1997 investigates ammonia metering adjustments made duringtransient emissions. A pulsed urea dosage device is disclosed which usesspeed and load engine sensor data read into a control unit to pulse aquantity of ammonia in stoichiometric relationship to NOx emissions, atsteady state conditions. During step-urea, step-load and transientcycles, the stoichiometric relationship was decreased and a dynamiccontrol strategy of injecting additional quantifies of urea after thetransient or step or load was completed was adopted. A reduction in NOxemissions is reported although it is questionable whether the reductionwas achieved because of the dynamic control strategy the additionalreductant added during, the transient or a combination thereof.

SAE paper 925022, “Catalytic Reduction of NOx in Diesel Exhaust” by S.Lepperhoff, S. Huthwohl and F. Pischinger, March, 1992 is an earlyarticle that looked at step load changes to evaluate transient systems.The article recognizes that when the load on the engine changed atconstant rpm, the NOx emissions increase, the temperature increases andthe total exhaust flow increases. Response of the catalyst to stepchanges in the engine operating conditions are referred to as step loadtests. Ammonia slip occurred when engine load increased and the articleconcludes the slip is correlated to the ammonia stored in the catalyst.It was suggested that a control program or control system would have toconsider the, NOx emissions of the engine, the catalyst temperature andthe ammonia stored within the catalyst, to avoid ammonia slip.

SAE paper 952493, “An Urea Lean NOx Catalyst System for Light DutyDiesel Vehicles” by H. Luders, R. Backes, G. Huthwohl, D. A. Ketcher, R.W. Horrocks, R. G. Hurley, and R. H. Hammerle, October, 1995 concludesthat an ammonia SCR system can control NOx diesel emissions. The controlstrategy used in the study was similar to that disclosed in the Gladdenand Lane patents above i.e., a microprocessor mapped engine out NOxemissions and catalytic converter temperature. Engine out NOx wasderived from engine speed and torque. Space velocity (intake air massflow) and catalyst temperature were then used with NOx out data to set amaximum NOx reduction rate. Transient operation was numerically modeledfrom steady state conditions. Ammonia storage and thermal inertia wasnoted as factors affecting the conversion but the control systemdiscussed had no special provisions, other than numerical modeling.

SAE paper 930363, “Off-Highway Exhaust Gas After-Treatment: CombiningUrea-SCR, Oxidation Catalysis and Traps” by H. T. Hug, A. Mayer and A.Hartenstein, March, 1993, describes stoichiometric injection of ammonia,without lag, based on engine mapped conditions. Catalyst porosity isstated to be important with respect to transient emissions. An injectionnozzle for metering is disclosed.

An article entitled “NOx—Reduction in Diesel Exhaust Gas with Urea andSelective Catalytic Reduction” by M. Koebel, M. Elsener and T. Marti,published in Combustion Science and Technology, Vol 121, pp. 85–102,1996 describe experiments conducted “at abrupt load changes”. An abruptreduction in load did not cause ammonia slip but an abrupt increase inload did cause ammonia slip. The article observes that the catalyst issaturated with adsorbed ammonia at lower temperatures; that increasedload significantly increases NOx emissions; that increased loadincreases, slowly, the temperature of the catalyst. Ammonia slipoccurring at the onset of the abrupt load change because of excessiveammonia present when the desorption of the ammonia is increased whilethe bulk at of the catalyst bed is too cool to effectively react thedesorbed ammonia with the higher level of NOx. This observation has beennoted in several of the prior art references discussed above. Therecommendation is to retard the addition of ammonia in relation to theload increase.

In general collective summary of the prior art references discussedabove, it is known that ammonia SCR systems can be used effectively tocontrol the emissions produced by vehicles powered by diesel engines;that the reducing catalysts adsorbs and stores ammonia at lowtemperatures and desorbs the stored ammonia at higher exhaust gastemperatures; that steady state NOx emissions, determined from mappedspeed and load engine conditions, can be readily controlled by meteringammonia in stoichiometric relationship to the NOx emissions; that it ispossible to pump urea, react urea to produce ammonia and preciselycontrol the rate of ammonia rejection to the exhaust gases bycontrolling pulsed injections of ammonia; and that transient emissionscause transient increases in NOx concentrations with attendant exhaustgas temperature increases requiring a reduction in the ammonia meteringrate to balance the increased ammonia present attributed to desorptionresulting from the temperature increase. Noticeably absent, from any ofthe mobile applications discussed, is a simple control system capable ofquickly and effectively adjusting the metering rate during transientemissions as well metering the reductant during steady state operatingconditions.

In this regard and as noted above, industrial processes, which do nothave the sudden transient emission changes of a vehicular application,can utilize NOx sensors in a closed loop controlled through set-pointcontrollers. There are no commercially available NOx sensors which havethe response time needed for vehicular applications. Thus any SCRcontrol system for mobile applications will necessarily be open loop.

SUMMARY OF THE INVENTION

Accordingly, it is a principal object of the present invention toprovide a control for an NOx SCR mobile emission reduction system whichis able to control the system to reduce transient as well as steadystate NOx emissions without reductant slip.

This object along with other features of the invention is achieved in amethod for reducing NOx emissions produced in mobile diesel applicationsby an external reductant supplied to an SCR system comprising the stepsof a) sensing one or more engine operating parameters to predict aconcentration of NOx emissions indicative of the actual quantity of NOxemissions produced by the engine; b) when the actual concentration ofNOx emissions changes and the temperature of said reducing catalyst iswithin a set range, varying the actual concentration of NOx emissions bya time constant to produce a calculated concentration of NOx emissionsdifferent than the actual concentration of NOx emissions; and, c)metering the external reductant to the reducing catalyst in said SCRsystem at a rate sufficient to cause the reducing catalyst to reducesaid calculated concentration of NOx emissions whereby metering of thereductant accounts for the effects on said SCR system attributed totransient NOx emissions. More particularly, the NOx constant acts todecrease the actual concentration of NOx emissions when the NOxemissions increase to avoid: reductant slip and increase the actual NOxemissions when the NOx emissions decrease to utilize the catalystreductant storage abilities.

In accordance with an important feature of the invention, the NOx timeconstant is a function of the catalyst temperature within a settemperature range as that catalyst temperature relates to the capacityof any given catalyst to store reductant at that temperature. Generally,the storage ability of the catalyst decreases as the catalysttemperature increases within the catalyst temperature range whereby thereductant is metered, during and following an NOx transient, on thebasis of the ability of any specific catalyst used in the SCR system tostore the reductant thus minimizing the likelihood of reductant slip.

It is a distinct feature of the invention that the ability of any givencatalyst to store reductant is expressed as the relative time it takesfor any given catalyst to store reductant at any given catalysttemperature to generate a varying time constant that can be accessedthrough a conventional look up table storing time constant-catalysttemperature relationships. The time constant is utilized to account forthe lag in the catalyst response to the NOx transient by modifying theNOx emission concentration in any number of ways, such as by determininga moving average of NOx emissions over varying time periods, each timeperiod correlated to a time constant in the look-up table for a thencurrent catalyst temperature, so that reductant dosage is determinedwithout having to sense numerous parameters and perform numerouscalculations to periodically determine current storage capacity of thecatalyst for setting the reductant metering rate.

However, it is a distinct feature of the invention to provide a filterto account for the lag in the catalyst system attributed to transientNOx emission by filtering the actual NOx emissions (increasing ordecreasing) to NOx concentrations which do not exceed the catalyst'sability to store reductant at its current temperature in a responsiveand robust control. In accordance with this feature of the invention,the filter uses the capacity of the catalyst to store reductant at thelower temperatures of the catalyst temperature range while alsoproviding, when the reductant is aqueous urea, improved urea hydrolysisby the provision of two first order filters in series represented in thecontinuous time domain by the transfer function:

${H(s)} = {\frac{1}{{\tau_{1} \cdot s} + 1} \cdot \frac{1}{{\tau_{2} \cdot s} + 1}}$andτ1=τ2=ƒ(Cat)When filtering the actual NOx emission concentration, the variable NOxtime constant, τNOx, is determined from the look-up table noted above asa function of the catalyst temperature. In accordance with the broaderscope of the invention, the second order filter is effective tointroduce a lag for any temperature dependent relationship of thecatalyst, including but not limited to those that are only straight lineor constant approximations of the catalyst's ability to store reductantat certain temperatures within a temperature range of the catalyst.

In accordance with another distinct aspect of the invention, the systememploys a second order filter, as represented in the continuous formdesignated above, to account for the changing heat fronts moving throughthe catalyst bed which are attributed to NOx transient emissions andproduces a functional catalyst temperature which is a more accuratetemperature than that achieved by sensing pre or post or mid-bedcatalyst temperatures. In accordance with this aspect of the invention,(which is not limited in application to control systems which factor NOxconcentrations but can be applied to any mobile system which measures orsenses catalyst temperature for any reason), the catalyst time-constantτCat is a function of the space velocity of the exhaust gases throughthe catalyst.

In accordance with this distinct feature of the invention, a method fordetermining the functional temperature of a catalyst in an exhaustsystem of a vehicle includes the steps of i) determining, by sensing orcalculating, the temperature of the exhaust gases and the space velocityof the exhaust gases through the catalyst and ii) filtering the exhaustgas temperature by a catalyst filter, to generate the functionaltemperature of the catalyst. Significantly, the catalyst filterimplements a time constant determined as a function of changing spacevelocity to filter the exhaust gas temperature and is implemented in thecontinuous time domain by a second order filter as set forth above.

In accordance with a still further feature of the invention, thetransfer function, H(s), for the NOx and catalyst filters of the presentinvention can be easily implemented in any number of discrete forms intothe vehicle's existing microprocessor because any of the conventionaldifference equations implementing the transfer function in discrete formare not memory intensive.

In accordance with yet another aspect of the invention, the two firstorder filters forming the functional catalyst temperature are split,upon engine shut-down, from a series relationship into two individualparallel operating first order filters with ambient temperature fed asfilter input to the temperature of the catalyst so that after shortengine stop/start periods, the cooled down temperature of the catalystis used in the second catalyst filter to prevent reductant slip afterengine restart. In accordance with this aspect of the invention, thecool down time constant, τCool, is determined as a function of timeelapsed from vehicle shut down or parameters that represent differencein temperature. By arranging both filters in parallel so that eachreceives the same information, the second parallel filter is preventedfrom freezing or drifting when the filters are switched back to seriesrelationship upon restart of the vehicle.

In accordance with a specific feature of the invention, the externalreductant is ammonia and the storage capacity of the reducing catalystwhich is used to set the time constant τNOx for any given catalyst is afunction of a) the surface area of the catalyst over which the exhaustgases flow, b) the number and strength of adsorption/absorption sites onthe surface area and c) the ability of the catalyst washcoat to storeNOx at any given temperature within the set temperature range whereby acontrol method not only uses a well known reductant to optimize theperformance of any given catalyst, but also provides a method tooptimally size a reducing catalyst for any given engine/vehiclecombination to, meet regulated drive cycle NOx emission requirements.

In accordance with a still further feature of the invention, theexternal reductant is metered at a normalized stoichiometric ratio ofreductant to NOx emission established for the current functionalcatalyst temperature (as determined by the catalyst NOx constant)whereby each essential step of the method, i.e., the NOx emissions, thecatalyst functional temperature and the NSR ratio, have all beenadjusted to account for the inevitable effects on the SCR systemresulting from engine operating conditions producing NOx transientemissions which otherwise adversely affects the operation of the SCRsystem. Again, the current state of the catalyst does not have to besensed nor intensive calculations run based on sensed catalyst state toset the reductant rate.

Still another specific and inclusive feature of the invention is toprovide a method for metering an external reductant to a reducingcatalyst in an SCR system applied to a vehicle powered by an internalcombustion engine which includes the steps of

a) sensing operating conditions of the vehicle and engine to generate,by calculation and/or measurement, signals indicative of the actualquantity of NOx emissions emitted by the engine, the temperature of theexhaust gas and the space velocity of the exhaust gas;

b) filtering, when the temperature of the catalyst is within a settemperature range, the actual NOx emission signal by an NOx timeconstant to produce a calculated NOx signal different than the actualNOx signal when the NOx signal is changing;

c) filtering the exhaust gas temperature signal by a catalyst timeconstant to produce a functional catalyst temperature signal differentthan the exhaust gas temperature when the space velocity signal changes;

d) factoring the functional catalyst temperature signal and the spacevelocity signal to generate a NSR signal indicative of a normalizedstoichiometric ratio of reductant to NOx emissions; and,

e) metering the reductant to the reducing catalyst by factoring thecalculated NOx signal by the NSR signal to produce a metering signalcontrolling a metering device for the external reductant.

It is a general object of the invention to provide a nitrogen based SCRcontrol system for NOx emissions produced by diesel powered vehicles.

It is another general object of the invention to provide an externalreductant SCR control system for mobile IC engine applications whichminimizes reductant slip while utilizing the ability of the SCR catalystto store reductant.

It is an object of the invention to provide a control system for mobileNOx SCR systems having any one or any combination of the followingcharacterizing features:

a) Ability to control reduction of transient as well as steady state NOxemissions;

b) Ability to prevent reductant slip during NOx transient emissions;

c) Simple to implement in programmable routines not subject to extensivememory requirements stored in the ECU;

d) Robust, stable and not subject to significant drift over time;

e) Able to account for thermal aging of catalyst;

f) Easily implemented in OBD diagnostic systems;

g) Inexpensive because it requires no additional parts or componentsother than what is currently used in state-of-art systems;

h) Not limited to any specific driving cycle or test cycle; and

i) Insensitive to arbitrary changes to temperature and/or load and/orNOx emissions.

Another distinct but related object of the invention to provide a methodfor determining the functional catalyst temperature of SCR catalysts foruse in any control type system resulting from changes attributed to NOxtransient emissions notwithstanding what methodology is used toestablish the catalyst temperature at steady state conditions.

Still another stand alone but related object of the invention is toprovide a method for ascertaining the start-up temperature of anycatalyst in any emission system.

Still yet another object of the invention is to provide a control systemfor a mobile IC engine SCR application which is able to account forchanges to the SCR system attributed to NOx transient emissionsnotwithstanding the fact that such control systems may employ NOx and/orreductant sensors assuming commercially acceptable, time responsivesensors are developed for mobile engine applications.

Still another object of the invention is to provide an SCR controlsystem for mobile IC applications using an external reductant which canfunction with any design or type of reducing catalyst used in the SCRsystem.

Another object of the invention is the provision of an SCR controlsystem for mobile IC applications using an external reductant whichprovides a basis for optimizing the selection of any specific reducingcatalyst for any given engine/vehicle combination.

Yet another object of the invention is to provide an SCR control systemfor an external reductant which determines and uses the storage/releasecapacity of reductant and NOx emissions for any given SCR catalyst tocontrol reductant metering in a manner that accounts for the capabilityof that specific catalyst to reduce NOx emissions as a result of NOxtransient emissions produced by the engine.

A still further object of the invention is the provision of a controlsystem for an external reductant applied to a mobile IC engine having anSCR system in which reductant usage is minimized while maintaining highNOx conversion.

Another object of the invention is to provide a control system for anexternal reductant SCR system which emulates the lag of the catalystfollowing NOx transient emissions by use two simple first order filtersin series having time constants determined as a function of temperature.

These and other objects, features and advantages of the presentinvention will become apparent to those skilled in the art upon readingand understanding the Detailed Description of the Invention set forthbelow taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in certain parts and an arrangement of partstaken together and in conjunction with the attached drawings which forma part of the invention and wherein:

FIG. 1 is a graph of NOx emissions produced in the exhaust gases of adiesel powered vehicle during the European MVEG cycle;

FIG. 2A depicts a series of graphs showing torque, inlet and outletcatalyst temperatures and NOx emissions produced over time during a stepload test of a diesel powered vehicle;

FIG. 2B depicts a series of graphs similar to FIG. 2A for a rapidlysequencing step load test;

FIG. 3 is a schematic representation of an SCR system controlled by themethod of the invention;

FIG. 4 is a flow chart indicative of the steps used in the presentinvention to control the SCR system;

FIG. 5 is a map of space velocity and catalyst temperature variablesresulting in various normalized stoichiometric ratios;

FIG. 6 is a schematic representation of the NOx filter of the presentinvention in a discrete form;

FIGS. 7A and 7B are schematic representations of first order filterssuitable for use in the present invention;

FIG. 7C is a graph depicting the filter effects of the filters shown inFIGS. 7A and 7B;

FIG. 8 is a graph depicting the filter effect of the invention on a stepload transient emission;

FIG. 9 is a graph of an ETC test plotting the functional catalysttemperature predicted by the invention, the sensed mid-bed catalysttemperature and the sensed average catalyst temperature;

FIG. 10 is a schematic representation of the TCat filter of the presentinvention in a discrete form;

FIG. 11 is a constructed graph indicative of the variable NOx constantused in the present invention, i.e., τNOx; and,

FIG. 12 is a graph of a portion of an ETC cycle showing ammonia slip forsystems which metered the reductant based on actual NOx emissionscompared to a system using the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein the showings are for the purposeof illustrating a preferred embodiment of the invention only and not forthe purpose of limiting the same, there is shown in FIG. 1 an MVEG testconducted with a test vehicle equipped with a 1.9 liter turbocharged,direct injection (TDI) diesel engine, i.e., a light duty test vehicle.The invention will be described throughout as applicable to a dieselengine, but as indicated above, the invention, in its broader sense, isapplicable to any internal combustion (IC) engine, such as gasoline fueltype engines operated lean or with “lean burn” engine fuel strategy.

In FIG. 1, the European MVEG cycle is plotted in seconds on the x-axiswith the nitrogen oxides (NOx) emitted by the vehicle during the drivecycle plotted on the left-hand y-axis and the vehicle speed plotted inkm/hr on the right-hand y-axis. The lower trace identified by thereference numeral 10 is a plot of the vehicle speed over the timedportion of the drive cycle. The uppermost plot identified by referencenumeral 12 are the NOx emissions produced by the diesel powered vehicleduring a regulated drive cycle and is characterized, ratherdramatically, by “spikes’ of NOx transient emissions. Several factualobservations concerning the graph of FIG. 1 should be noted as follows:

1) When the vehicle is traveling at a constant speed, the NOx emissionsare somewhat constant. This can be shown, for example, by looking atthat portion of the vehicle speed plot designated by reference numeral10A and comparing it to the generally flat portion of NOx emissionsgenerated during that time period in the NOx plot section designated byreference numeral 12A. Assuming load is constant, constant vehicle speedwill produce constant or “steady state” NOx emissions. As discussed inthe Background, NOx control strategies in use today are based on steadystate conditions. Thus when an SCR system uses an external reductant,the external reductant is metered, under today's strategies, at a ratesufficient to reduce the NOx constant emissions produced at 12A. If thespeed (and load) of the vehicle remain fairly constant, as in mostindustrial processes, high NOx conversion efficiencies can be obtained.FIG. 1 shows why that control philosophy can not work with a mobile ICengine application.

2) When the vehicle accelerates, such as indicated by the accelerationdesignated as reference numeral 10B, the NOx emissions correspondinglyand dramatically increase or “spike” as shown by spike 12B and the spikeor pulse or increase in NOx emissions is commonly referred to as an NOxtransient. Further the faster the acceleration (or rate at which load isapplied to the vehicle), the greater the NOx transient emission. Thetime scale used in FIG. 1 does not permit scrutiny of each of thetransient pulses or “spikes”. However, each transient has a leading edgeand a trailing edge. The leading edge which at its peak defines theamplitude of the transient emission inherently occurs as an inevitableresult of the combustion process within the engine. The trailing edgeexhibits a more gradual decline. The drive cycle of FIG. 1 was performedon a diesel powered vehicle with exhaust gas recirculation (EGR).Advances in the micro-processor art coupled with known computer-basedtechniques such as feed forward, artificial intelligence, adaptivelearning, etc., have resulted in faster responding EGR systems which actto dampen the trailing edge of the transient. However, the leading edgeof the transient will always occur. The expected advances in the EGR artconceivably could result in a lowering of the overall NOxconcentrations. However, such systems will not replace the controlsystem of the present invention or the need for a control method such asdescribed herein. That is, improved EGR systems will supplement thecontrol method described herein to assure full functionality of the SCRsystem using an external reductant, over the entire operating range ofthe engine.

3) When the vehicle decelerates, such as at the deceleration designatedby reference numeral 10C, the NOx emissions drop and drop below the NOxconcentration which occurs at steady state conditions such as indicatedby the corresponding NOx emission drop shown by reference numeral 12C inFIG. 1.

In general summary, FIG. 1 shows the NOx transient emissions comprise asignificant portion of the NOx emissions emitted by a diesel poweredvehicle during a regulated drive cycle.

As a matter of definition, and when used in this Detailed Descriptionand in the claims:

a) “Deceleration” means an engine operating condition whereat thevehicle motors the engine.

b) “Acceleration” encompasses a rate of change in the engine systemwhich is increasing and is not limited merely to rate changes in theengine rpm but also includes i) increases in engine load whether or notaccompanied by a change in engine speed and ii) changes in operatingparameter(s) of the total engine “system” such as for example by achange in EGR flow or composition or actuation of a turbo charger.

c) “NOx transient” means a temporary increase in NOx emissions asexplained with reference to FIG. 1, and by “definition”, occurs at anacceleration.

d) “Steady state” means constant and occurs when the engine operatingparameters or the engine “system” does not significantly change over adiscrete time period, i.e., say NOx varies ±5%.

e) “SCR” means selective catalytic reduction and includes a reducingcatalyst(s) which speeds or enhances a chemical reduction of NOx by achemical reagent.

f) “External reductant” means any reducing reagent which is supplied tothe exhaust stream from a source other than the products of combustionproduced in the combustion process of the engine. In the preferredembodiment, the external reductant is a nitrogen based reagent such asammonia metered in liquid or gaseous form to the reducing catalyst.However, as indicated above, the control of the present invention isalso capable of functioning with other reductants such as fuel oil.

g) “Normalized stoichiometric ratio” or “NSR” is defined as the molarquantity of reductant injected to the reducing catalyst divided by thetheoretical molar quantity of reductant which is needed to completelyreduce NOx. Setting the NSR<1 or sub-stoichiometric is a common practiceto avoid reductant slip.

h) “Space velocity” is the volumetric flow rate of the exhaust gas atstandard conditions (one atmosphere and 20° C.) divided by the volume ofthe catalyst, i.e., vol of gas [m³/hr]/volume of catalyst=−1 hr.⁻¹.

i) “Storage capacity” is the ability of a reducing catalyst to adsorb orstore a reductant and/or NOx emissions on its surface at a giventemperature.

Referring now to FIG. 2A, there is shown a portion of a Europeansteady-state cycle (ESC), i.e., a heavy duty test. This cycle stepsthrough a series of constant load and speed combinations. These datashown were taken from a 12 liter heavy duty diesel engine which did nothave an EGR system. The catalyst system was supplied from EngelhardCorp. and included a reducing or deNox catalyst and an oxidationcatalyst. A urea external reductant was applied in a conventional mannerto be described below. Control of the reductant was by a separatemicroprocessor which control communicated with the engine control unit(ECU). The control metered the reductant in a manner designated hereinas “NOx following” which means the reductant was metered based on theactual NOx emissions emitted by the engine as predicted by conventionaltechnology using steady state NOx maps of selected engine operatingparameters and the catalyst inlet temperature. The data shown in FIG. 2Ais thus somewhat typical of what occurs in conventional controltechniques which sense the engine operating parameters to determinesteady-state NOx emissions and then meter an external reductant into theexhaust gas at an NSR ratio sufficient to cause reduction of the NOxemissions as they pass over the reducing catalyst.

In FIG. 2A several plots or traces are shown over a timed portion of anESC cycle shown as seconds on the x-axis. The lowest most trace,designated by reference numeral 20, plots the load or torque on they-axis. It can be readily seen that the load is applied in steps withthe test including a relative high step load designated by referencenumeral 20A, followed by a series of 4 slightly varying, intermediateload steps followed by a relative low load step designated by referencenumeral 20B.

The uppermost trace of FIG. 2A is a plot of the NOx emissions in ppm(parts per million) emitted for the step loads imposed on the engineshown by trace 20. The emission scale for the plot is limited to 1000ppm of NOx and the emissions produced during step load cycle 20A is “offthe graph”. The remaining trace is within the scale and basicallycorrelates the known fact that engine loadings result in NOx emissionsattributed to the load. Note that the NOx concentrations basicallycorrespond to the torque on the engine.

The middle trace of FIG. 2A is plot of the exhaust gas temperature atthe inlet of the reducing catalyst designated by the trace passingthrough squares and indicated as reference numeral 24 and a plot of theexhaust gas temperature at the outlet of the reducing catalystdesignated by the trace passing through circles and indicated asreference numeral 25. Two observations should be noted. First, for“significant” step load changes, i.e., 20A and 20B, the outlettemperature of the catalyst significantly lags the inlet temperature ofthe catalyst and this lag can be hundreds of seconds. On the other handwhen the step loads do not significantly vary as shown by the mid bandportion of load trace 20, the outlet temperature follows the inlettemperature of the catalyst.

Referring now to FIG. 2B, there is shown the results of an especiallyconstructed step load using the same engine and reductant controlstrategy discussed with reference to FIG. 2A. In FIG. 2B, a series of 5rapidly applied step loads are applied to the engine followed by a “noload” period as shown by the lowermost trace designated by referencenumeral 27. The five rapidly applied step loads can be viewed asindicative of engine loads applied as the operator shifts thetransmission of a vehicle through various gear ranges. As expected, theuppermost trace of FIG. 2B designated by reference numeral 28 showsincrease and decrease of NOx emissions corresponding to the rapidchanges in load trace 27.

As in FIG. 2A, the middle portion of FIG. 2B shows a plot of exhaust gastemperature at the inlet, of the reducing catalyst passing throughsquares and designated by reference numeral 29 and a plot of exhaust gastemperature at the outlet of reducing catalyst passing through circlesand designated by reference numeral 30. Catalyst inlet and temperatureplot 29 clearly shows a rapid exhaust gas temperature increase duringthe transient NOx emission. Temperature plot 30 shows a periodic delayor lag before the exhaust gas temperature at the outlet “catches up” tothe inlet temperature. FIG. 2B shows that as a result of changes to theoperating conditions of the engine, the catalyst inlet temperature andNOx concentration rise and fall following the change in engine operatingconditions. However, a periodic time delay occurs before the entirereducing catalyst is affected by the change in exhaust gas temperaturecaused by the NOx transient emission. This is admittedly difficult todiscern in the ESC step load tests illustrated in FIG. 2A and still yetmore difficult to discern in a drive cycle such as shown in FIG. 1.However, this relationship, demonstrated in the specially constructedtests of FIG. 2B, forms one of the underpinnings of the presentinvention.

More specifically, a reducing catalyst, preferably one formulated toreact with a nitrogen reagent, has an ability to store and release thereductant and heat. The NOx emission is, for all intents and purposes,instantaneously formed and passes through the system at the speed of theexhaust gas. If the catalyst has sufficient stored reductant, thetransient NOx emission will react with the reductant and be reduced. Thetime constant for the NOx transient to pass through the catalyst bed ismuch smaller (i.e., the speed of the gas) than the time constant for thetemperature pulse to pass through the catalyst bed. As the temperatureof the reducing catalyst increases, its ability to store reductant andNOx decreases. Conventional control systems meter the reductant on thebasis of NOx emissions currently produced. FIG. 2B clearly shows that itis not only possible but likely that any given NOx transient can occurwhile the temperature of the reducing catalyst is experiencing the aftereffects of a prior NOx transient and is within a temperature rangewhereat the reductant rate called for by the current NOx transient cannot be stored on the catalyst. The inevitable result is reductant slip.The present invention addresses this problem, as explained in detailbelow, by accounting for the ability of the catalyst, at any given time,to store and release the reductant and NOx emissions with respect to theNOx emissions being currently produced by the engine.

Referring now to FIG. 3, there is shown schematically the elementscomprising an external reductant supplied SCR system applied to a mobileinternal combustion engine 32. An intake valve 33 controls admission ofair to the engine's combustion chamber 34 from an intake manifold 36. Anexhaust valve 37 controls the emission of exhaust gases produced to anexhaust manifold 38, in turn connected to an exhaust pipe 39. Attachedto exhaust pipe is a close-coupled catalyst 40 followed by a reducing orde-Nox catalyst 42 in turn followed by an oxidation catalyst 43.

All catalysts shown, 40, 42, 43, are conventional. While reducingcatalyst 42 is a necessary element for the inventive system to work, itis not, per se, part of the invention. That is the inventive method andsystem does not require specially formulated catalysts to function.Reducing catalyst 42 may generally comprise a zeolite or a mixture oftitanium, vanadium, tungsten and/or molybdenum oxides and one or morereducing catalysts may be used or different catalyst bed formulationsused in one reducing catalyst. Reference may be had to Byrne U.S. Pat.No. 4,961,917 incorporated herein by reference for a description of asuitable reducing catalyst. Generally, the oxidation catalyst comprisesa support such as alumina and a precious metal such as platinum forexample. As is generally known, oxidation catalyst 43 is provided, amongother reasons, to oxidize the excess of any unreacted ammonia leavingreducing catalyst 42 with oxygen to nitrogen and water. Reference may behad to Speronello et al. U.S. Pat. Nos. 5,624,981 and 5,516,497,incorporated herein by reference, for a staged metal promoted zeolitecatalysts having a stage favoring oxidation of excess ammonia. Closecoupled catalyst 40 is designed to reduce emissions during cold startand reference may be had to Sung U.S. Pat. No. 5,948,723, incorporatedherein by reference for a description of a catalyst suitable for coldstart engine applications.

A fuel injector 45 receives pressurized fuel from a fuel tank 46 forpulse metering of the fuel into combustion chamber 34. A fuel demandcommand, i.e., accelerator pedal, schematically represented by referencenumeral 48 controls fueling and consequently the speed/load of thevehicle.

In the embodiment of FIG. 3, an aqueous urea reservoir 50 stores aurea/water solution aboard the vehicle which is pumped through a pump 51including a filter and pressure regulator to a urea injector 54. Ureainjector 54 is a mixing chamber which receives pressure regulated air online 55 which is pulsed by a control valve to urea injector 54. Anatomized urea/water/air solution results which is pulse injected througha nozzle 56 into exhaust pipe 39 upstream of reducing catalyst 42.

This invention is not limited to the aqueous urea metering arrangementshown in FIG. 3. It is contemplated that a gaseous nitrogen basedreagent will be utilized. For example, a urea or cyanuric acid prillinjector can meter solid pellets of urea to a chamber heated by theexhaust gas to gasify the solid reductant (sublimation temperature rangeof about 300 to 400° C.). Cyanuric acid will gasify to isocyanic acid(HNCO) while urea will gasify to ammonia and HNCO. With eitherreductant, a hydrolysis catalyst can be provided in the chamber and aslip stream of the exhaust gas metered into the chamber (the exhaust gascontains sufficient water vapor) to hydrolyze (temperatures of about 150to 350° C.) HNCO and produce ammonia.

In addition to urea and cyanuric acid other nitrogen based reducingreagents or reductants especially suitable for use in the control systemof the present invention includes ammelide, ammeline, ammonium cyanate,biuret, isocyanic acid, melamine, tricyanourea, and mixtures of anynumber of these. However, the invention in a broader sense is notlimited to nitrogen based reductants but can include any reductantcontaining HC such as distillate fuels including alcohols, ethers,organo-nitro compounds and the like (e.g., methanol, ethanol, diethylether etc) and various amines and their salts (especially theircarbonates), including guanidine, methyl amine carbonate,hexamethylamine, etc.

The operation of engine 32 is under the control of an ECU (enginecontrol unit) 60. ECU 60 is a microprocessor based control systemcontaining a conventional CPU with RAM, nonvolatile RAM, ROM, look-uptables for engine mapping purposes, etc. ECU 60 receives input sensorsignal information, processes the data by programmed routines andgenerates actuator output signals. While a dedicated processor could besupplied to control the SCR system of the present invention, it is aparticular feature of the invention that the control system can functionwith existing sensors and the ECU now used to control the operation ofengine 32.

Conventional sensor input signals now processed by ECU 60 and utilizedin the current control system include a speed/load signal from aspeed/load pickup 61 (i.e., a speed sensor and a torque sensor such asused in the engine's transmission), a fuel demand signal fromaccelerator pedal 48, air intake temperature signal (optionally andadditionally humidity) from a temperature sensor 62, mass air flow froma pressure or air flow sensor 64 and optionally an exhaust gastemperature sensor from a temperature sensor 65 if the vehicle is soequipped. (Alternatively, exhaust gas temperature can be modeled by ECU60 from other sensors not shown such as ambient temperature, coolanttemperature, fueling signals. See the '064 patent incorporated byreference herein.) If additional sensors are to be supplied, suchsensors would take the form of a catalyst inlet exhaust gas sensor 67 oralternatively, catalyst inlet exhaust gas sensor 67 and a catalystoutlet exhaust gas sensor 68 for calculating mid-bed steady statetemperatures of reducing catalyst 42. As indicated, ECU 60 reads thesensor input signals, performs programmed routines typically involvinglook-up tables to access mapped data and generates output or actuatorsignals such as fuel injector actuator signal shown on dash line 69.Insofar as the control system of the present invention is concerned, ECU60 generates a metering signal on dash line 70 to urea injector 54.

Referring now to FIG. 4, there is shown a flow chart of the presentcontrol system which is conventional in the sense that engine operatingparameters are read to determine a NOx concentration of emissionsproduced by the engine at engine out NOx block 75; a normalizedstoichiometric ratio of reductant to NOx emissions is set at NSR block76; a reductant rate is set based on the NSR and engine out NOxemissions at reductant calc. block 77 and urea injector 54 is instructedat injection block 78 to meter the atomized urea/water mixture at adefined pulse based on reductant calc. block 77.

In particular, engine out NOx block 75 receives a speed signal 80 and atorque signal 81 from speed/load sensor 61 at any given time andaccesses a map stored in a look up table in ECU 60 to predict for thattime the actual NOx emissions emitted by engine 34. Reference can be hadto SAE paper 921673, incorporated by reference herein, for severaltorque/speed maps. While some prior art has suggested mapping NOxtransient emissions, the suddenness of the transient coupled withvarying decay coupled further with overlapping or compound transients isbelieved to either render the mapping not feasible or the mapping wouldbe so extensive that a significant increase in memory of ECU 60 would berequired.

Within the prior art, the NSR ratio at NSR block 76 has been simply beenset at some fixed sub-stoichiometric ratio to minimize reductant slip orbreakthrough. Others, recognize that competing oxidation reactions athigher catalyst temperatures (among other reasons) adjust the NSR ratioto reflect a sub-stoichiometric NSR at current catalyst temperature.Still others, additionally recognize that the residence time or contacttime (i.e., space velocity) of the reductant with the catalyst has to besufficient to allow the reductant to diffuse and react with the NOx butstate that the system design or system operation is such that theresidence time is not a limiting factor. These prior art systemsdetermine the NOx concentration (predicted open loop from steady stateengine maps) and meter the reductant at a rate determined by the NSR,established in the manner indicated, applied (i.e., multiplied) to theNOx concentration.

This invention follows a generally similar approach in that aconcentration of NOx is determined and a NSR value is applied to the NOxconcentration to determine the rate at which the reductant is meteredupstream of reducing catalyst 42. However, the NSR is extrapolated froma stored look-up table which sets a NSR value which will not produce areductant slip above a set reductant range, i.e., say 5 ppm ammonia or 3ppm ammonia. The actual slip concentration is set at or below a or anexpected regulatory standard. An example of an NSR map used in thepresent invention and stored in a look-up table to be accessed by ECU 60is contained in FIG. 5. In FIG. 5, catalyst temperature is plotted onthe z-axis and encompasses a temperature range at which reducingcatalyst 42 can reduce NOx emissions, typically 200–400° C. butgenerally 150–600° C. The space velocity is plotted on the x-axis,typically anywhere from 5 to 60,000 hr.⁻¹, and the NSR ratio, plottedfrom a value in slight: excess of 1 to zero, (producing a reductant slipwithin a defined range) is plotted on the y-axis. Note the significantvariation in NSR as a result of variations in space velocities andcatalyst temperatures.

By way of explanation, reference can be had to an article “NOx Removalwith Combined Selective Catalytic Reduction and Selective NoncatalyticReduction: Pilot-Scale Test Results” by B. Gullett, P. Groff, M. LindaLin and J. M. Chen appearing in the October, 1994 issue of Journal ofAir & Waste Management Association, pages 1188–1194, (the “Pilot-TestArticle), incorporated by reference herein and made a part hereof. Theauthors of the Pilot-Test Article had earlier presented a paper entitled“Pilot-Scale Testing of NOx Removal with Combined Selective CatalyticReduction and Selective Non-Catalytic Reduction” at the 1993 JointEPA/EPRI Symposium on Stationary Combustion NOx Control, Bal Harbour,Fla. (the “Paper”) also incorporated by reference herein and made a parthereof. Both the Paper and the Pilot Test Article discuss an industrialNOx reduction system using a hybrid SNCR/SCR system which neverthelessdrew several conclusions relative to the SCR system which have beenutilized and expanded in this invention. The Pilot-Test Articlerecognized that space velocity is a controlling factor but determinedthat space velocity did not affect the performance of the SCR catalystwhen the stoichiometric ratio of ammonia to NOx was set at 1 or less.The Paper discussed high space velocities (encompassing velocitiescapable of being achieved by mobile applications) but concluded thestoichiometric ratio was not significantly influenced by the high spacevelocities. The Pilot-Test Article also recognized that there was anoptimum temperature range for the SNCR system. However, the article didconclude that ammonia slip can be determined on the basis of thestoichiometric ratio of ammonia/NOx and is independent of the quantityof NOx, i.e., FIG. 7 of the Pilot-Test Article. That conclusion hassince been verified for mobile SCR applications to which this inventionrelates. However, it has been determined that space velocity does affectammonia slip for any given NSR of 1 or less as well as temperature(within the set temperature range). Importantly, it has been determinedthat a relationship exists between NSR, space velocity and catalysttemperature which will produce a reductant slip of a set concentration.Accordingly, it is possible to model space velocity, catalysttemperature and NSR for any practical slip concentration to establish aNSR value at any given sensed (or modeled or computed) catalysttemperature and space velocity from which a rate at which the reductantis metered can be established without producing a reductant slip thatexceeds a set amount. Although a mathematical model could be generated,in practice, the NSR values are determined empirically for any givenreducing catalyst and mapped as shown in FIG. 5. Establishing a range ofNSR values, which is substantially sub-stoichiometric, producing no morethan a set concentration of reductant slip as a function of spacevelocity and catalyst temperature within a set range is believeddifferent than that used in other mobile SCR control systems and forms apart of the present invention. It should be understood that theinvention disclosed herein will function using any conventional priorart system for setting the NSR. Improved results are possible using themethod of establishing a NSR value vis-a-vis FIG. 5. Still further, anyconventional SCR control system relying on steady state conditions toestablish metering of the reductant or any other system to establish theconcentration of NOx emissions to be reduced, can be improved if the NSRis determined at a set slip range by modeling space velocity, catalysttemperature and NSR for that range.

As described thus far, this method will produce very high NOx conversionefficiencies sufficient to meet increasingly stringent NOx regulationsbut only for steady state engine operating conditions.

For reasons discussed above with reference to FIGS. 2A and 2B, the aftereffects of the NOx transient are not felt until a relatively long andvariable time has elapsed. While the median temperature of the catalystbed defines the ability of the catalyst bed, the precise position in thebed at which a median temperature can be sensed changes as the effectsof the transient emissions are experienced by the catalyst. Because ofthe transient wave front moving through the bed even if temperatureupstream and downstream of the reducing catalyst was simultaneouslysensed and applied to some formula giving a mid-bed temperature (forexample, the temperature summed divided by two) an instantaneoustemperature indicative of the median bed temperature would not result.Apart from considerations relating to the temperature of the reducingcatalyst, it is generally known that conventional steady state systemscause reductant slip on acceleration. This results because the transientcalls for a corresponding high amount of reductant while the catalyst ispartially or fully charged with stored reductant and the catalyst bedtemperature is substantially less than the inlet temperature, so thatslip inevitably occurs. After the transient the engine NOx emissionsdrop and the steady state system calls for a reduction in the reductant.However, the transient has already passed through the reducing catalystand the stored reductant used so that the reducing catalyst is ready tostore the reductant but the system calls for minimum reductant.

This invention introduces a delay in the control system to account forthe after effects of NOx transients while still allowing steady-statecontrol to occur. A delay is introduced in the actual emissionsdetermined in engine out NOx block 75 by an NOx filter block 90 and adelay is introduced in the catalyst temperature signal 91 (such assensed by temperature sensors 65, or 67, or 67 and 68, or modeled fromengine operating parameters) by a Cat filter block 92. Each filter block90, 92 changes its input signal in a diminishing manner over a lag ordelay period correlated to the time constant used in the filter. Inaccordance with the broader scope of the invention, a non-varying or anarbitrary time constant can be utilized for the NOx filter on the basisthat any diminishing delay of the signal is better than none. However,as will be explained, a particularly important feature of the inventionis that the NOx filter employs a varying time constant correlated to thecapacity of the catalyst to reduce NOx emissions at a given temperature.

NOx calculated filter 90 is a second order filter which is designed tomake use of the NH3 storage capabilities of the SCR system at lowtemperature and to improve urea hydrolysis, as well. The time constantfor this second filter is extrapolated from a look-up table correlatingthe capacity of the catalyst to store reductant at a given temperatureas explained further below.

The theory behind the second order NOx filter is as follows. Twoconsiderations are important: a) NH₃ storage in the catalyst increaseswith decreasing catalyst temperature, and b) hydrolysis reaction (ureasolution -> NH₃ molecules) improves with decreasing space velocity.

Considering actual (engine out NOx block 75) and filtered (NOx filterblock 90) NOx signals during a step load cycle, for example, thefiltered NO_(x) signal is smoother. This also leads to a smootherinjection profile. When taking-off engine load the filtered signal fallsdown slower than the actual NOx. At the same time the space velocity islow, so a high NSR value will be taken from the NSR look-up table. Thisprovides the opportunity to inject additional urea under“hydrolysis-friendly” conditions. The additional urea quantity islimited by the storage capacity of the catalyst in turn correlated tothe catalyst temperature and space velocity of the exhaust gas.Therefore, a filter dependent on such correlation is introduced.

This filter comprises two first order filters in series with both timeconstants being a function of the catalyst capacity at any givencatalyst functional temperature. In other words the time constants areequal to each other and are variable. The value is obtained from alook-up table which has a functional catalyst temperature as input andfrom which a time constant related to the capacity of the storereductant at that temperature is established as an output. In thecontinuous time domain the filter (two first order filters in series)can be represented by the transfer function [ratio of the transform ofthe output variable NOx_(Filt), (NOx filter block 90) to the transformof the input variable NOx_(EngOut) (engine out NOx block 75)] shown asequation 1:

Equation  1:${H_{NOx}(s)} = {\frac{{NOx}_{Filt}}{{NOx}_{EngOut}} = {\frac{1}{{\tau_{NOx1} \cdot s} + 1} \cdot \frac{1}{{\tau_{NOx2} \cdot s} + 1}}}$andτNOx1=τNOx2=ƒ(Cat)where:

-   NOx_(EngOut)=Predicted engine out NOx (from map, block 75)-   NOx_(Filt)=Filtered NOx-   τ_(NOx1), τ_(NOx2)=Time constant−function of catalyst temperature    correlated to capacity of catalyst to store reductant and NOx-   s=differential operator (continuous domain)    note:    “H _(NOx) (s)” is a general representation of a transfer function,    “H”. The “NOx” subscript indicates the process and the “(s)” points    out that it is a continuous process. The form “s” should be    recognized as a differentiation term which means that the correction    to the input occurs only during changing or transient conditions.

However this equation is implemented in a controller which means thatthis formula has been put in a discreet form, i.e., differenceequations. In a discreet form the first, first order filter may berepresented by equation 2:

$\begin{matrix}{{{NOx}_{Filt1}(n)} = {\frac{{{NOx}_{EngOut}(n)} - {{NOx}_{Filt1}\left( {n - 1} \right)}}{\tau_{NOx1}} + {{NOx}_{Filt1}\left( {n - 1} \right)}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$But because two first order filters have been placed in series, theactual filtered NOx value is the result of the second first orderfilter, which in discreet formula may be represented by differenceequation 3:

$\begin{matrix}{{{NOx}_{Filt2}(n)} = {\frac{{{NOx}_{Filt1}(n)} - {{NOx}_{Filt2}\left( {n - 1} \right)}}{\tau_{Nox2}} + {{NOx}_{Filt2}\left( {n - 1} \right)}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$andτNOx1=τNOx2=ƒ(Cat)where:

-   NOx_(EngOut)=Predicted engine out NOx (block 75);-   NOx_(Filt1)=NOx value after first filter, intermediate value;-   NOx_(Filt2)=NOx value after second filter, final filtered NOx value;-   τ_(NOx1), τ_(NOx2)=Time constant, function of catalyst capacity at    predicted temperature.    Note:

The subscripts “Filt1” and “Filt2” indicate values of the first orsecond first order filter. The subscript “(n)” indicates the value ofthe current sample. The subscript “(n−1)” indicates the value of theprevious sample. The output of Equation 2 is substituted for the inputin Equation 3. The time interval at which samples n are taken can bearbitrarily set or is a function of the speed of the processor at itcompletes a loop through the programmed routine. In the preferredembodiment, the time interval at which the algorithm is performed is setnear to the operating speed of ECU 60.

Reference can be had, by way of further explanation, to FIG. 6 whichshows NOx filter block 90 in discrete form comprising a first 1st orderfilter 90A corresponding to equation 2 and a second 1st order filter 90Bcorresponding to equation 3. Input to first 1st order filter 90A is theoutput from engine out NOx block 75 and a time constant shown as τNOx online 84. Output from first 1st order filter 90A is fed as input on line85 as is time constant τNOx to second 1st order filter 90B.

Those skilled in the art will recognize that the invention in itsdiscrete form is not limited to equations 2 and/or 3 but can encompassany number of conventional first order filters. Reference can be had toFIG. 7A which in schematic form represents a first order filter of thetype defined by equations 2 and/or 3. By way of explanation relative toNOx filter 90, input designated “x” in first calculation block 82 isτNOx, τ, and actual emissions from engine out NOx block 75, μ₁. Feedbackis μ₂. An integration block 83 performs a conversion factor from thenumber of samples to time (i.e., seconds). Reference should be had toFIG. 7B which shows an alternate discreet 1st order filter, employingthe same terminology and reference numerals. The functioning of bothfirst order filters is generally shown in FIG. 7C in which the input isrepresented by line designated 87, generally indicative of a step load.The output of 1st order filter shown in FIG. 7A is designated byreference numeral 7A and output of 1st order filter shown in FIG. 7B isdesignated by reference numeral 7B.

Reference can now be had to FIG. 8 which is an input/output graphdepicting the various filter concepts falling within the broader scopeof the invention. Limiting discussion to NOx filter 90 (althoughapplicable to Cat filter 92), an input indicative of a step load(discussed with reference to FIG. 2) is shown by the straight lineindicated by reference numeral 93. The y-axis can be viewed as NOxconcentrations and the x-axis time so input 93 is the actualconcentrations of NOx produced by the engine as predicted from look-upmaps at engine out block 75. Again, NOx filter block 90 can take, inaccordance with broader aspects of the invention, various forms. Afilter based on a moving average as discussed above is represented bydash line 94 and clearly shows a change in the concentration of thepredicted actual emissions over a lag period which change diminishesover the filter delay period. A first order filter, only, such as shownin FIGS. 7A and 7B is shown by the output curve passing through squaresand designated by reference numeral 95. The preferred second orderfilter is shown by the output curve passing through circles anddesignated by reference numeral 96. As will be explained below, the NOxtime constant for all filters, is determined by a look-up table whichestablishes, for various temperatures of any specific catalyst, a timeconstant indicative of the time response of the catalyst to storereductant and NOx emissions as a function of the capacity of thecatalyst.

As can be seen in FIG. 8 the load increase and the load decrease isdelayed. A delay in adding the reductant at the onset of the transientreduces the likelihood of reductant slip. A delay in recognizing adecrease in NOx emissions results in an increase in reductant but at acondition, i.e., lower exhaust speed and temperature, whereat thehydrolysis reaction is more likely to proceed to completion and thereducing catalyst storage capacity is increased.

Referring still to FIG. 4, a filter is applied to determine thetemperature of reducing catalyst 42 in order to account for the changingtemperature of the catalyst associated with the NOx transient emission.The exhaust gas velocity on input 98 such as determined by air mass flowor pressure sensor 64 is converted to a current time space velocity atspace velocity block 99. Space velocity output as shown is to NSRcalculation block 76 per the map as discussed above relative to FIG. 5.However, the space velocity is also used to filter the catalysttemperature.

Conceptually, the catalyst temperature prediction can be effected by useof a running average of a measured exhaust gas which conceptually can beeither post or pre catalyst. The number of samples of temperature takenfrom which the average catalyst temperature would be determined wouldthen depend on the value of the space velocity. The reasoning behindthis concept was that with a step load increase or decrease a heat orcold front, respectively, moves through the catalyst. The speed at whichthis front proceeds through the catalyst is space velocity dependent.

While in accordance with the broader concepts of the invention, a movingaverage for the catalyst mid-bed temperature can be used (as well as amoving average for calculated NOx emissions with the number of samples,dependent on the storage capacity of the catalyst at its current mid-bedtemperature), it is a specific aspect of the invention that a secondorder filter can be used to predict catalyst temperature because, amongother reasons, the second order filter is far more easier to implementthan a mathematical technique to determine a moving average over avarying time integral which may be significantly long in duration. Againthe implementation includes two first order filters in series withvariable time constants. The time constants are a function of the spacevelocity through the catalyst. In transfer function form, the filter canbe defined by equation 4:

$\begin{matrix}{{H_{TCat}(s)} = {\frac{{TCat}_{Filt}}{TExhaust} = {\frac{1}{{\tau_{Cat1} \cdot s} + 1} \cdot \frac{1}{{\tau_{Cat2} \cdot s} + 1}}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$andτCat1=τCat2=ƒ(SV)where:

-   TCat_(Filt)=Predicted catalyst temperature;-   TExhaust=is the measured exhaust gas temperature upstream of the    catalyst;-   τ_(Cat1), τ_(Cat2)=Time constant which is a function of the space    velocity;-   s=differential operator (continuous domain)    note:    “H _(TCat) (s)” is a general representation of a transfer function,    “H”. The “TCat” subscript indicates the process and the “(s)” points    out that it is a continuous process. Again, the form “s” should be    recognized as a differentiation term which means that the correction    to the input occurs only during changing or transient conditions.

Again this equation is implemented in a controller which means that thisformula has been put in a discreet form. In a discreet form the first,first order filter may be defined by equation 5:

$\begin{matrix}{{{TCat}_{Filt1}(n)} = {\frac{{{TExhaust}(n)} - {{TCat}_{Filt1}\left( {n - 1} \right)}}{\tau_{Cat1}} + {{TCat}_{Filt1}\left( {n - 1} \right)}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

But because two first order filters have been placed in series in orderto create a second order filter, the actual filtered catalysttemperature is the result of the second first order filter, as shown bydiscreet or difference equation 6:

$\begin{matrix}{{{TCat}_{Filt2}(n)} = {\frac{{{TCat}_{Filt1}(n)} - {{TCat}_{Filt2}\left( {n - 1} \right)}}{\tau_{Cat2}} + {{TCat}_{Filt2}\left( {n - 1} \right)}}} & {{Equation}\mspace{14mu} 6}\end{matrix}$andτCat1=τCat2=f(SV)where:

-   TExhaust=Measured exhaust temperature upstream of the catalyst;-   TCat_(Filt1)=Catalyst temperature after first filter, intermediate    temperature;-   TCat_(Filt2)=Predicted catalyst temperature, final filtered    temperature;-   τ_(Cat1), τ_(Cat2)=Time constant which is a function of the space    velocity.    note:

The subscripts “Filt1” and “Filt2” indicate values of the first orsecond first order filter. The subscript “(n)” indicates the value ofthe current sample. The subscript “(n−1)” indicates the value of theprevious sample.

The time constant for the catalyst, τCAT, is constantly varying(straight line) indicated schematically by the graph shown in FIG. 4designated by reference numeral 97 and generally determined inaccordance with equation 8 as follows:

$\begin{matrix}{\tau_{CAT} = \frac{M_{CAT} \cdot {Cp}_{CAT}}{{SV} \cdot P_{GAS} \cdot {Cp}_{GAS} \cdot V_{CAT}}} & {{Equation}\mspace{14mu} 7}\end{matrix}$where

-   M is the mass of the catalyst;-   V is the volume of the catalyst;-   Cp_(CAT) is the heat capacity of the catalyst;-   P_(GAS) is the density of exhaust gas;-   Cp_(GAS) is the heat capacity of the exhaust gas; and,-   SV is the space velocity of exhaust gas.

Equations 6 and 7 provide a method to determine in real time thetemperature of the reducing catalyst accounting for the temperaturechange attributed to NOx transient emissions.

While it is a necessary element of the present invention, thedetermination of the catalyst temperature by a filter using a spacevelocity time constant, τCat, may be used in other emission controlsystems or for other emission related functions. Although a thermocouplecan directly measure temperature, the life of a thermocouple in avehicular environment is limited. More importantly, a moving heat wavefront(s) resulting from a NOx transient emission(s) does not uniformlydissipate its heat to the catalyst bed. A precise position can not beestablished for all the transients whereat a maxima or average heatfront will occur. Generally, a mid-bed temperature is necessary toestablish an accurate NSR but the wave front position is variable andnot strictly speaking at the exact mid-point of the catalyst. Whileinlet and outlet exhaust gas temperature sensors 67, 68 can givereadings with difference divided by a constant, i.e., two, to give amid-bed approximation, significantly more accurate results are obtainedif the catalyst temperature (½ the sum of inlet and outlet temperatures)is filtered by the τCat constant to account for the changing nature ofthe heat wave front passing through the catalyst. In fact, τCat has beenfound to accurately predict catalyst temperatures using only the exhausttemperature at inlet sensor 67 or exhaust manifold sensor 65 (or even amodeled exhaust gas temperature from ECU 60). As used herein and in theclaims, reference to catalyst functional or functioning temperaturesmeans the mean or median temperature of the reducing catalyst bed. Thecatalyst filter using a τCat constant to modify precatalyst exhaust gastemperature produces a more accurate functional catalyst temperatureaccounting for transient emission heat wave fronts than other knownmethods. In support of this, reference should be had to FIG. 9 which isa graph of catalyst temperatures taken during an ETC (European transientcycle) test. The trace passing through diamonds and designated byreference numeral 87 is the average catalyst temperature recorded byinlet and outlet sensors 67, 68. It should be recalled that FIG. 2B (and2A) plotted the inlet and outlet temperatures separately. The inlettemperature oscillated significantly and the outlet temperature was“damped”. Neither inlet nor inlet temperature readings can account forthe catalyst temperature when affected by transient emissions. Averagecatalyst temperature trace 87, as expected, dampens the inlettemperature oscillations. However, the oscillations are still presentand demonstrate why actual temperature measurements are not suitable fora reductant metering system. The mid-bed temperature was measured duringthe ETC test and is plotted as the trace passing through squaresdesignated by reference numeral 88 in FIG. 7. The mid-bed temperaturedoes not have the oscillations associated with the exhaust gas, and asexpected, has a wave form shape. The predicted temperature, determinedby τCat, is shown by the trace passing through circles designated byreference numeral 89. The predicted temperature was based on sensingexhaust gas temperature only. Predicted temperature trace 89 followsmid-bed temperature trace 88 and clearly demonstrates why it is asuperior tool, easily implemented in any control system withoutintensive heat transfer calculations to determine the functionalcatalyst temperature.

A particularly novel feature of this filter is that during a shortengine stop the filter predicts the catalyst temperature according tothe cooling down process. This is done by splitting the two first orderfilters in series into two individual parallel operating first orderfilters and feeding the ambient temperature into both filters as input.The output of the filters represents the predicted catalyst temperature.The reason that the combined filters are split is because a cooling downcurve of a catalyst can be represented very well by a first order filterwith a variable time constant. The time constant is computed from alook-up table which has as input, e.g., a timer or the difference inpredicted catalyst temperature and ambient temperature. The transferfunction in continuous form for this filter is represented by equation8:

$\begin{matrix}{{H\;{\tau_{Cool}(s)}} = {\frac{T_{Cool}}{T_{Amb}} = \frac{1}{{\tau_{{{Cool}\; 1},2} \cdot s} + 1}}} & {{Equation}\mspace{14mu} 8}\end{matrix}$andτCool1=τCool2=ƒ(Timer)where:

-   TAmb=Ambient temperature-   TCool=Catalyst temperature while cooling down-   τ_(Cool1), τ_(Cool2)=Time constant which is a function of time,    e.g., timer or temperature difference

Reference should be had to FIG. 10 which shows in schematic, conceptualform, how the cool down filter can be implemented in Cat filter block92. A switching arrangement is shown to switch the filters from a seriesarrangement to a parallel arrangement. The filter is shown switched intoits series arrangement and operates with τCat and exhaust gastemperature inputs to the first 1st order filter 92A and then to thesecond 1st order filter 92B in the same manner as explained withreference to FIG. 6. On engine shut down, switch line 103 actuates theswitches to the position shown by the dotted lines. Switching inputsτCool and ambient temperature on line 104 to second 1st order filter 92Bto predict the cool down temperature of the catalyst used upon enginerestart.

The reason for feeding the second parallel placed filter with the sameinformation as well is to keep this filter (i.e., first 1st order filter92A) from freezing or drifting to faulty values when the engine isstarted again after a short stop. On restart, the filters are connectedagain in series and both have to start from the same values in order topredict accurately the catalyst temperature again based on the measuredvalue.

The advantage of using the temperature difference between computedcatalyst temperature and ambient temperature for determination of thetime constant is that no counters are required which have to have acapacity for dealing with “large” times.

This feature is not only very useful during testing but also in practicewhen a vehicle has made a short stop (refueling, etc.). The software isnot starting from a default (reset) value. Starting from a default valuewhile the catalyst is hot means that urea is not injected at theearliest possible moment, which means lower conversions.

The above representation of the catalyst temperature filters is for thecontinuous domain. However they are implemented in discreet form in thecontroller by difference equation 9 as follows:

$\begin{matrix}{{TCool}_{{{Filt}\; 1},2} = {\frac{{{TAmb}(n)} - {{TCool}_{{{Filt}\; 1},2}\left( {n - 1} \right)}}{\tau_{{{Cool}\; 1},2}} + {{TCool}_{{{Filt}\; 1},2}\left( {n - 1} \right)}}} & {{Equation}\mspace{14mu} 9}\end{matrix}$andτCool1=τCool2=f(Timer)where:

-   TAmb=Ambient temperature;-   TCool_(Filt1,2)=Predicted catalyst temperature while cooling down,    both filters;-   τ_(Cool1)=τ_(Cool2)=Time constant which is a function of e.g. timer    or temperature difference    note:

The subscript “Filt1,2” indicate values of parallel operating first andsecond first order filter. The subscript “(n)” indicates the value ofthe current sample. The subscript “(n−1)” indicates the value of theprevious sample.

It is to be appreciated that the use of a first order filter to predictthe cool down temperature of the catalyst after engine shut-off is afeature that can be implemented in any emission system whether or not afunctional catalyst temperature is obtained by the invention. In suchapplication only a first order filter is used to generate a cool downtemperature of the catalyst and the cool down temperature can be usedfor any purpose needed by the system. The cooled own time constant,τCool, would still be generated by a look-up table either a timingconstant based on catalyst temperature on shut down or a table based onthe difference between catalyst temperature at shut down compared toambient temperature. The catalyst temperature would be the temperaturecalculated by the system using the cool down filter. In its broaderinventive scope, the cool down filter is not limited to a reducingcatalyst. For example, a hybrid vehicle employing an engine whichintermittently shuts off and on, could use a catalyst temperature forfueling control which can be easily attained by the first order cooldown filter.

The NOx filter constant, τNOx, is specific to the reducing catalyst usedin the SCR system. It is determined as a function of the ability of anyspecific reducing catalyst to store (and release) reductant at any giventemperature (within the reductant storage temperature range of thereducing catalyst). Preferably, the NOx filter constant is determined asa function of the ability of any specific reducing catalyst to store theexternal reductant and NOx emissions (within the catalyst temperaturerange).

Reference should be had to FIG. 11 which is a graph of the τNOx timeconstant for various functional catalyst temperatures (as determinedfrom Cat filter block 92) of a specific reducing catalyst 42. Catalystfunctional temperature is plotted on the x-axis and τNOx is plotted as atime constant on the y-axis. The plotted time constant was establishedthrough a series of tests. The catalyst was purged of any reductant andheated to a temperature within the known catalyst storage range (about200 to 400° C.). An inert gas with a set concentration of reductant wasmetered through the catalyst and gas samples taken at time increments.When the gas samples showed a reductant concentration approximatelyequal to that metered into the reducing catalyst, a storage time wasestablished and plotted on the y-axis for that temperature. Theresulting plot designated by reference numeral 100 is then stored in alook-up table within ECU 60. What this procedure did was to relate thestorage capacity of the catalyst to a time period which time periodvaries with the temperature of the catalyst and is used to set the timeconstant τNOx for the NOx filter.

The time period while relative in the sense of an absolute time quantityaffords a consistent basis for establishing the time constant for allcatalysts because a catalyst having a greater capacity to storereductant than another catalyst will, for any given temperature, take alonger time before it stops storing reductant (metered at a setconcentration rate) than a catalyst having less storage capacity. Bycorrelating the catalyst storage capacity to a storage time, a basisexists for determining the timeliness of the catalyst to react tochanges in NOx emission concentrations. All that is needed is todetermine the change in emissions because the catalyst behavior inresponse to the change can be predicted. The control system does nothave to monitor operating parameters to ascertain how the catalyst isresponding in real time to the changes nor perform any number ofintensive calculations based on the catalyst response (in the endcorrelated to the capacity of the catalyst) to adjust the reductantdosage.

While sufficient tests have not been conducted as of the date of thisapplication, it is believed that data taken from a number of τNOx curvesgenerated from different reducing catalysts will establish a correlationbetween the number and strength of storage/release sites on the surfacearea of the reducing catalyst and the τNOx curve for that specificreducing catalyst. In the preferred ammonia reductant embodiment, thereducing catalyst can be formulated to produce, per unit area, a meannumber of Bronsted acid sites having a mean bond strength at whichammonia molecules attach. The τNOx curve is then generated for theformulated washcoat with known reactivity on the basis of the surfacearea of the catalyst i.e., the larger the surface area, the longer timeor larger τNOx constants. Because the τNOx curve is selected to matchthe transient emissions generated by any given engine, a method fordetermining catalyst sizing matched to a specific engine is established.Catalyst cost is minimized while NOx regulations are met.

It is also known that reducing catalysts (particularly zeolites)similarly have a varying affinity to store (and release) NOx emissionsdepending on the temperature of the catalysts. The inventioncontemplates producing a more accurate τNOx constant by accounting forthe NOx storage capacity of the reducing catalyst in a manner similar tothat explained above which determined and measured the reductant storagecapacity of the reducing catalyst. The τNOx constant is thus establishedon the capacity of any given reducing catalyst to store the reductantand NOx.

The inventive method as thus described senses exhaust gas velocity atinput 98 to produce a space velocity signal at space velocity block 99which is inputted to NSR block 76. Space velocity signal is also used toaccess a catalyst time constant in τCat look up table 101 which timeconstant is inputted to Cat filter block 92 to generate a functionalcatalyst temperature, adjusted for NOx transients, which is inputted toNSR block 76 for determining a NSR ratio pursuant to the map of FIG. 5.The catalyst temperature is also used to access a NOx time constant, ina τNOx look up table 102, which is inputted to NOx filter block 90 togenerate a calculated NOx emission concentration which accounts for thetransient NOx emissions produced by engine 32. The NSR ratio andcalculated or filtered NOx emissions are inputted to Reductant calcblock 77 which determine the concentration of reductant to be injectedupstream of reducing catalyst 42 at injection block 78 which controlspulse metering of the reductant. Note that all major system components,NOx, temperature, NSR and reductant calculation have all been adjustedfor the effects attributed to transient emissions.

It should be apparent that the inventive system, is matching the delayexperienced in real life from sudden changes in engine operatingconditions on the ability of the reducing catalyst to reduce NOxemissions by delaying the impact of the NOx emissions which wouldotherwise be sensed and used to control reductant metering. The delay isa relative but consistent number, τNox, in the sense that it is based onthe relative ability of the catalyst to reduce NOx emissions (i.e.,storage capacity) at varying temperatures. Further the catalysttemperature is a real time prediction based on the delay of the catalystto experience the changing exhaust gas wave front. The metering is setin accordance with a varying NSR (predicted current functional catalysttemperature and current space velocity). The system thus accounts fortransient emissions by emulating the effects of the transient emission,temperature and NOx, without any attempt to measure catalyst performancefollowed by an adjustment of reductant metering rate such as disclosed,for example, in the '186 patent.

As described, the control system is fully functional. There are howeverseveral enhancements, additions or modifications which can be made tothe control system to improve its overall operation. In particular arate of temperature change control shown as dT/dt block 110 can beincorporated into the system. While the algorithms discussed aboveinherently account for increasing/decreasing values in the sense ofpositive and negative numbers, a time derivative of catalyst temperaturecan establish, over longer time periods, a decreasing or increasingtemperature change attributed to NOx transients. When and depending onthe rate of temperature change determined by dT/dt block 110, a variableconstant can be applied to the τNOx constant on a decrease intemperature rate allowing asymmetric τNOx constants for increasing anddecreasing NOx emission rates. Further, dT/dt block 110 can sense adeceleration and stop reductant metering at block 77 when a decelerationoccurs. In this regard, it should be recognized that when the catalysttemperature is below the catalyst range (i.e., less than 150° C.) thereis no reductant metering. There is no reductant metering upon a vehicledeceleration. Also, when the catalyst temperature is above the catalysttemperature range (i.e. greater than about 400° C.), the reductant ismetered at a rate equal to the actual NOx emissions produced asdetermined by engine out NOx block 75. At such higher temperature theNOx time constant, τNOx, has a value of 1. Again, it is to be noted thatthe NOx filter is operating during changing conditions within a settemperature range whereat the catalyst has ability to store reductant.The fact that the filtered NOx emissions used to set reductant dosage inthis range may, on acceleration, be less than engine but emissions hasno bearing on the reduction of the NOx transient but has a bearing onreductant slip.

The system can also account for catalyst ageing by inputting an ageingfactor at input 111 to the NSR signal generated at NSR block 76. Theageing signal may be accessed from a look up table which correlateseither engine hours of operation or miles driven taken from vehiclesensors to an ageing factor which modifies the NSR ratio.

If time responsive, commercially acceptable NOx sensors are developedsuitable for vehicle application, these NOx sensors would be used inplace of steady state engine maps to determine the actual concentrationof NOx emissions at engine out NOx block 75. If time responsive,commercially acceptable reductant, i.e., ammonia, sensors are developed,these sensors would be used to trim the reductant signal at reductantcalculation block 77.

The system can also include provision for on board diagnostics (OBD)which can be implemented, for example, by a NOx or reductant sensordownstream of reducing catalyst 42. As discussed above, current NOxand/or ammonia sensors do not have adequate response times for controlof a mobile IC application. They are satisfactory for diagnosticpurposes however.

Reference can be made to FIG. 12 which shows two traces of reductantslip (ammonia) recorded over an ETC drive cycle with the reductantmetered to the SCR system with and without the control system of thepresent invention. Examination of FIG. 12 shows that contained within anouter trace 120 is an inner trace 121. Outer trace 120 represents aconventional metering control referred to as “NOx following” indescribing FIG. 2A, i.e., reductant metered on the basis of actualpredicted actual NOx emissions emitted by the engine as determined bysteady state NOx emission maps from engine operating parameters andcatalyst temperature. The inner trace 121 plots the reductant slip whenthe same engine is operated under the control system for the ETC cycle.In all cases the reductant slip is reduced for transient emissions. Inthe tests conducted when data for inner trace 121 was gathered, thesystem was not equipped with the dT/dt differential temperature changeblock 110 to determine decelerations. Based on a review of the slip datait is believed that if dT/dt block 110 was implemented in the control tostop reductant metering during vehicle decelerations, a furthersignificant reduction in slip would occur.

The invention has been described with reference to a preferred andalternative embodiments of the invention. Modifications and alterationsto the invention will occur to those skilled in the art upon reading andunderstanding the Detailed Description of the Invention set forthherein. It is intended to include all such modifications and alterationsinsofar as they come within the scope of the present invention.

1. A method for controlling dosage of an external reductant supplied toan SCR system including a reducing catalyst, in a mobile applicationhaving an IC engine to account for transient NOx emissions produced bythe engine, said method comprising the steps of: a) determining theactual NOx emissions produced by the engine to generate an NOx actualsignal; b) filtering the NOx actual signal when the reducing catalyst iswithin a set temperature change to produce a variable delay in the NOxactual signal; and c) metering the reductant in real time at the valueof the delayed NOx actual signal.
 2. The method of claim 1 where thefiltering step is effected by an NOx filter having an NOx time constantdetermined as a function of the capacity of the reducing catalyst at thetemperature of the catalyst.
 3. The method of claim 2 wherein said timeconstant is a variable correlated to the time any given catalyst canstore and/or release a given quantity of reductant at a given catalysttemperature within said set catalyst temperature range, said storagetime decreasing as said catalyst temperature increases within said settemperature range.
 4. The method of claim 2 wherein said NOx filter in acontinuous time domain filters the actual concentration of NOx emissionsin accordance with the transfer function${H(s)} = {\frac{1}{{\tau_{1} \cdot s} + 1} \cdot \frac{1}{{\tau_{2} \cdot s} + 1}}$where τ₁ equals τ₂ and is a function of the operation of said reducingcatalyst.
 5. The method of claim 4 wherein said NOx filter is indiscreet form and implemented by a microprocessor, said NOx filter insaid discrete form including two first order filters in series with oneanother.
 6. The method of claim 5 wherein the first NOx filter indiscrete form is represented by diffeence equation${{NOx}_{Filt1}(n)} = {\frac{{{NOx}_{EngOut}(n)} - {{NOx}_{Filt1}\left( {n - 1} \right)}}{\tau_{NOx1}} + {{NOx}_{Filt1}\left( {n - 1} \right)}}$and the second NOx filter in discrete form is represented by differenceequation${{NOx}_{Filt2}(n)} = {\frac{{{NOx}_{Filt1}(n)} - {{NOx}_{Filt2}\left( {n - 1} \right)}}{\tau_{NOx2}} + {{NOx}_{Filt2}\left( {n - 1} \right)}}$where τ_(NOx1) equals τ_(N0x2); NOx_(EngOut) is said actual NOxemissions; NOx_(Filt1) is the NOx value after said first filter and isan intermediate value, and NOx_(filt2) is said calculated NOx emissions.7. The method of claim 6 further including the step of determining thetemperature of said reducing catalyst by sensing or calculating thetemperature of the exhaust gas and filtering the temperature of theexhaust gas by a catalyst filter when the space velocity of the exhaustgas changes to establish a functional catalyst temperature.
 8. Themethod of claim 7 wherein said catalyst filter in the continuous timedomain is represented by the transfer function.${H(s)} = {\frac{1}{{\tau_{1} \cdot s} + 1} \cdot \frac{1}{{\tau_{2} \cdot s} + 1}}$where τ₁ equals τ₂ and is a function of the space velocity of theexhaust gases.
 9. The method of claim 8 wherein said catalyst filter isin discreet form and implemented by a microprocessor, said filter insaid discrete form including two first order filters in series with oneanother.
 10. The method of claim 9 wherein the first catalyst filter indiscrete form is represented by difference equation${{TCat}_{Filt1}(n)} = {\frac{{{TExhaust}(n)} - {{TCat}_{Filt1}\left( {n - 1} \right)}}{\tau_{Cat1}} + {{TCat}_{Filt1}\left( {n - 1} \right)}}$and the second catalyst filter in discrete form is represented bydifference equation${{TCat}_{Filt2}(n)} = {\frac{{{TCat}_{Filt1}(n)} - {{TCat}_{Filt2}\left( {n - 1} \right)}}{\tau_{Cat2}} + {{TCat}_{Fil2}\left( {n - 1} \right)}}$where τ_(CAT1) equals τ_(CAT2); TExhaust is said exhaust gastemperature; TCat_(Filt1) is the catalyst temperature after firstfilter, intermediate temperature and TCat_(Filt2) is said functionalcatalyst temperature.
 11. The method of claim 10 wherein$\tau_{CAT} = \frac{M_{CAT} \cdot {Cp}_{CAT}}{{SV} \cdot P_{GAS} \cdot {Cp}_{GAS} \cdot V_{CAT}}$where M is the mass of the catalyst; V is the volume of the catalyst;Cp_(CAT) is the heat capacity of the catalyst; P_(GAS) is the density ofexhaust gas; Cp_(GAS) is the heat capacity of the exhaust gas; and, SVis the space velocity of exhaust gas.
 12. The method of claim 1 furtherincluding the steps of sensing ambient temperature after shut down ofthe engine and filtering the catalyst temperature by a first orderfilter using a cool down time constant established as a function of timeelapsed from engine shut down at any given catalyst temperature at shutdown or as a function of the difference in temperature between ambienttemperature and catalyst shut down temperature whereby the catalysttemperatute upon engine restart is determined.